Gas-fired residential and commercial water heaters are generally formed to include a vertical cylindrical water storage tank with a gas burner disposed in a combustion chamber below the tank. The burner is supplied with fuel gas through a valved gas supply line, and combustion air through an air inlet flow path providing communication between the exterior of the water heater and the interior of the combustion chamber.
Water heaters of this general type are extremely safe and quite reliable in operation. When gasoline or other flammable liquids are stored or used improperly in proximity to the water heater, however, it is possible that flammable vapors may become entrained in the air intake of the water heater. Such vapors might cause secondary combustion to occur within the confines of the water heater combustion chamber.
In residences where gas water heaters are located in close proximity to gasoline containers, there is a potential for the gasoline vapors to be ignited by an ignition device, such as, for example, the water heater pilot flame. A vapor sensor mounted to the water heater would shut down the pilot if gasoline vapors are detected. Present sensors capable of detecting flammable vapors can be rendered ineffective or deliver false  alarms when the sensing element is damaged, exposed to air flow, dust, liquids, oils, or other contaminants. The sensor may also be rendered ineffective if deliberately blocked so that vapors cannot reach the sense elements.
In view of the foregoing, a continuing need exists for the preclusion of fuel flow to the appliance when extraneous flammable vapors are present exteriorly adjacent the appliance. In particular, a continuing need exists for improved gas vapor sensors, which are resistant to contamination and also tampering. 